Add an option to enable building with -fvisibility=hidden.

When this option is set gtest itself is built with -fvisibility=hidden
with gcc and clang.  This replicates MSVC's default behaviour with gcc
and clang.

googletest/CMakeLists.txt

@@ -22,6 +22,11 @@ option(gtest_build_samples "Build gtest's sample programs." OFF)
 
 option(gtest_disable_pthreads "Disable uses of pthreads in gtest." OFF)
 
+option(
+  gtest_hide_internal_symbols
+  "Build gtest with internal symbols hidden in shared libraries."
+  OFF)
+
 # Defines pre_project_set_up_hermetic_build() and set_up_hermetic_build().
 include(cmake/hermetic_build.cmake OPTIONAL)
 
@@ -46,6 +51,11 @@ if (COMMAND set_up_hermetic_build)
   set_up_hermetic_build()
 endif()
 
+if (gtest_hide_internal_symbols)
+  set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+  set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
+endif()
+
 # Define helper functions and macros used by Google Test.
 include(cmake/internal_utils.cmake)